What is normal idle speed and how do I read it?

2001, 4.0l, automatic, 4X4, ~80K miles

Noticed a few weeks ago, a slight vibration when coming to a stop. After this went on for sometime, I noticed the Tach needle was lower than normal.

On the tach, there are two hash marks between 0 and 1. All other markings are three hash marks between numbers. I always assumed the first hash mark under the 1 was a reading of 750 RPMs where it normally used to idle. Now it idles between the two hash marks 500 RPMs :dunno:.

So I have searched and jeepforum for similar symptoms, which I can find but I cannot find a clear solution to my low idle behavior. By the way, even came across a few post suggesting older 4.0l engines do that and folks should just live with it. I am not really satisfied with that response so I'm giving you all a crack at it.

Problem:
-Cold start, engine revs to ~1200 RPMs.
-Linearly as it warms up to normal operating temp, the idle slowly drops down to just under the first hash mark under the 1 on the tach. And of course it has those normal little random stumbles that pop the revs a ~hundred RPM then settles back down(but I'm not worried about the slight stumble).
-Trans in drive with A/C activated it will idle at ~750 RPM.
-Trans in drive with A/C deactivated idle drops down to ~500 RPM.
-Anything above idle and she runs like a top.
-No CEL codes.

What I've done so far:
-Cleaned IAC and seat. Reset PCM to re-zero steps settings per FSM.
-Cleaned Throttle Body while off the intake manifold.
-Checked for vacuum leaks...none found.
-Replaced brooken rear CCV. guess that was a vacuum leak but made no change in idle.
-New spark plugs to Autolite 3924's... more snappy throttle now. Old plugs were only about 2 years 20K miles old and looked like normal wear without any obvious tale tale warning signs.
-Checked TPS resistance. Ohmed at 980 TB closed and about 3700 WOT. Very smooth transition with no dropouts.
-Tested both upstream O2 sensors. Heaters are good and output voltage varied while running and is between 0.2v-0.9v. did not check down stream sensors.
-Removed every ground I could find in the engine bay and cleaned along with battery and cables.
-Checked Coolant temperature sensor and Manifold Air Temperature sensor. They both have normal resistance readings for the appropriate temps.
-Tried to check Intake/Exhaust manifold bolts. All the bolts seem to be tightly rust frozen in place an I cant really get any of my torque wrenches in there to verify.
-I did buy a new IAC thinking the existing one may be slow to respond or have a bad spot when warmed but it does change idle speed at start up and when A/C is activated/deactivated so I haven't put it in yet.

Is there anything I have overlooked or should I just live with it as some others have suggested? I would like to take the IAC back if I don't actually need it.

TIA jeepers

Idle speed should be 700-750 rpm, a/c OFF, transmission in DRIVE, engine and transmission fully warmed up--after about 20 minutes of operation.

Don't have the specs for your year XJ on the TPS, but if you used a digital ohm meter the buffer will mask problems, you need to use an analog ohm meter when checking the TPS range of movement.

Yes, I did use a DVM (Fluke 25) older model but very high quality for its time. Has an instant gradiant display bar along with the digital readout. It's supposed to mimick the analog meters for fast changes. Not to sure about the buffer causing reading problems

So my idle is low per the tach?

Well, if you are at 500 rpm in DRIVE with the a/c off, everything at normal operating temp, then yes. Sorry you shafted with such a poor tach. You might be able to rent one at the parts store to get a decent reading.

If you bought a scanner I think it would give you an accurate rpm reading.

Is it suggested then that the tach is good at startup then gets poor after it warms up? I don't really follow that...

I do have a code reader (not a scanner) but it doesn't give me RPM readings. That is a good idea to check the tach with something else.

But I do think there is still a problem with the idle RPMs being low enough to get a vibration regardless of accuracy of the tach.

Greenz said:

Is it suggested then that the tach is good at startup then gets poor after it warms up? I don't really follow that...

I do have a code reader (not a scanner) but it doesn't give me RPM readings. That is a good idea to check the tach with something else.

But I do think there is still a problem with the idle RPMs being low enough to get a vibration regardless of accuracy of the tach.

Click to expand...

When the rpms drop low enough you are on the verge of a stall, pretty much stretching the capabilities of the electronics to correctly compensate. Rent or borrow a tach and recheck the rpms.

Thanks Joe for the tip. I have an old school Tach/Dwell meter but I don't think it will work with the coil rail setup. I will keep looking.

Also a non Jeeper I work with suggested that I have a "Lazy IAC". Works most of the time but has a spot or two (step) that just isn't stopping in the right spot. He said he has a drawer full of them and that I would be able to test that theory. Apparently that model IAC fits other applications.

I will keep you all updated once I find the fix.

please do, i have the exact problem as you and have been unable to find a cause

what engine sensors are still stock on your jeep??

Fred85 said:

what engine sensors are still stock on your jeep??

Click to expand...

All sensors are still stock.

Update:

Disconnected both battery cables and touched cables together for 1 minute to reset ECM. Did this after reading another post to correctly reset ECM. Results equal no apparent change in idle speed. Previously reset the ECM by leaving it disconnected from the battery for about three hours.

Took the IAC out and moved the pintle in and out by hand a few times (even pulled the pintle, spring, and shaft guard off the motor). I felt it catch in a couple of spots. Put old IAC back in and reset the ECM to re-tune it. I was curious to see how easily it would get damaged. Still functions the same way so moving the pintle by hand didn't easily damage it.

Went ahead and put the brand new IAC in, reset ECM, and still no change in the way it's running. Still have the low idle with no real way of verifying RPMs against the tach. Guess I have a new IAC spare now.

Ordered a Scangauge II and it should be here tomorrow. I will then be able to verify the engine RPMs against the tach. Hopefully the real-time updates on the sensors will help in troubleshooting this elusive problem.

Based on your update then can we a**ume that the idle does not change if the IAC is plugged in or unplugged?

If that is correct, then you should test the IAC wiring.

The IAC was thought to be "lazy" because idle speed would change (up and down) under certain conditions (A/C on/off, neutral to drive, cold start to warmed engine) but idle speed settles seemingly low on the tach.

With the IAC disconnected and the above mentioned conditions changed, the idle speed would change but more like a bogged down change. I think the ECM was trying to compensate for the no change in the IAC pintle position by changing timing or injector pulse widths.

I did get the Scangauge 2 in today and I was just out messing around with it. So, according to the Scangauge, The first hash mark below the one is indeed the 750 RPM mark. My idle when warm is between 650-680 RPMs (no A/C, and in park). water temp is 202*F. looks as if the Jeep temp and tach are closely matching Scangauge.

Disturbingly what I am noticing is ,TPS at idle is mostly at 0 but fluctuates with a 1 and 19 in the readout. I don't fully understand if that is normal or possibly the source of the problem.

my 01 XJ IDLES AT 750...MY TPS READS A STEADY 13 AT IDLE...

McQue said:

my 01 XJ IDLES AT 750...MY TPS READS A STEADY 13 AT IDLE...

Click to expand...

Sounds like a pretty good clue!

McQue said:

my 01 XJ IDLES AT 750...MY TPS READS A STEADY 13 AT IDLE...

Click to expand...

Is that with the Scangauge?

Read with my AutoXray Scanner...it read outs realtime data as well as codes

Any updates on the low idle greenz??

i was wondering if you replaced the TPS....

update?
I have the same issue on an '01 but haven't touched the IAC

Fred85 said:

Any updates on the low idle greenz??

i was wondering if you replaced the TPS....

Click to expand...

Haven't had a chance to work on it the last few days due to work.

I have been driving it back and forth to work and keeping an eye on the Scangauge readouts trying to get a feel for what is normal.

Every Scangauge readout seems to be normal with the exception of the TPS as mentioned earlier.

At startup, TPS readout flickers about once every 5 seconds or so. Gradually till fully warmed the TPS flickering increases to about once or twice per second. Fully warmed engine is when the low idle is happening. Also wanted to mention that I see the flickering at all positions of the TPS range not just at idle. Example of TPS: Idle = 0 but flicker reading is 18, Cruising 40MPH = 16 but flicker reading is 27. There is a larger difference at idle than off idle.

I highly suspect the TPS circuit is the root cause of this problem but I'm not running out to buy more parts until I know for sure. My next step will be to test all the wiring from the TPS back to the PCM. Looking at the FSM, there are 3 connections between TPS pin(1) and PCM C1 pin(4).

I once had a Dakota that decided to not start one day. After troubleshooting (no FSM) for about 4 hrs I found a severely corroded ground buried in the wiring harness that controlled the ASR. With that in the back of my mind it makes be believe it is possible to have something similar going on. I'm not into splicing in new grounds just yet because if there is a bad connection somewhere if could affect something else later down the road.

As mentioned earlier, I checked TPS resistance off the TB and is was smooth throughout the range of operation. This leads me to believe the TPS itself is ok.

My theory is there is an intermittent connection (increased resistance) somewhere in the TPS circuit that the PCM views as a very brief increase in throttle position. PCM compensates by altering the injector pulse-width and/or timing resulting in the low idle. It would make sense that off idle/driving, ~100 RPM fluctuation would be much less noticeable if at all.

One more thing, this problem too could be contributing to an increase in fuel consumption and slight richness issue it is having.

you are clearly a troubleshooter... I suggest you test/check/maybe even replace your coolant temp sensor for proper resistance output as it it wams ups as marginal performance can affect MPG ...just a thought